Hello i own a fiat punto 75s recently i started to do a full check for the cooling system because my car started to over heat .
Anyways my car kept overheating in really hot days (40C +).
But in long distance roads it wont reach it optimal temp ( 90C ) because i dont have a thermostat.
Everything is working fine besides that .. the radiator is not plugged, the fan kicks in about 92-95c , paraflu cooling water .
So my question is should i install a thermostat ? i afraid it will block the water entrance and reduce the cooling effect when it is a hot day .

so car was overheating with old thermostat? maybe it was stuck closed, did you try another one or did you just hammer it out from housing?
you can block half of the radiator with cardboard and look if car will warm up faster and remove it when its to hot

there is no thermostat at all , just a hole .

By overheating do you mean its creeping up over 95, say to 100-110? or going full on overheating until you have to stop the engine to avoid damage?
If its the prior install a thermostat and try flushing the coolant and refilling, make sure you bleed the system so there isn't any air in it.

I installed a new thermostat but the engine started to suffocated at low rpm .
after i installed the thermostat i turned on the engine at the first time it started to make noises of air suction from the Throttle Body while the rpm started to drop from 1k to below 1k .. off and on .

the hunting idle has nothing to do with the thermostat - hunting idle tends to mean there is a air leak on the intake after the butterfly of the throttle body, perhaps you have disturbed something..

You should have a thermostat yes, the car will take an absolute age to warm up and probably not really ever get to operating temperature unless you are sat in traffic for prolonged periods. This will result in it running rich nearly all the time, so increased fuel use, will coke up the spark plugs. Generally a bodge fix removing the stat internals and should be avoided. You want to get a decent thermostat however, not just the cheapest you can find. This is an item I always fit a genuine fiat part. Partsworld-UK on ebay sell genuine fiat stats and deliver worldwide - here is a link http://www.ebay.co.uk/itm/GENUINE-F...a-Uno-Seicento-Cinquecento-Fire-/271108178705

It should be totally fine to drive in the meantime, the only long term issue is fouled spark plugs as i mentioned before from running rich. So I would get the thermostat, fit that - take the plugs out and clean them and gap them at the same time, or just fit new ones if you don't know how old they are or if they look in not great condition. And make sure you don't have a split vacuum hose or one knocked off to sort the hunting idle, could also be leaking between the throttle body and the inlet manifold. Just go through everything make sure its tight and pipes are nice and snug and not cracked etc.

Everything is working fine now , with the new thermostat .
i think water came into the spark area , but now its dry everything is ok.